1. Field of the Invention
The invention relates to a system for demodulating a multiple tone Link-11 Signal, stripping off the Doppler component and locating the synchronization frame boundaries.
2. Background of the Invention
It is important to maintain continuous secure communication over a computer-to-computer radio link during military operations. Typically, a base Net Control Station (NCS), located on an aircraft carrier, keeps in constant contact with moving aircraft or surface vessels referred to as pickets or Participating Units (PU's). As the PU move towards and away from the aircraft carrier, Doppler shifts and other aberrations may be introduced. Various prior art devices and methods have been proposed to overcome these problems.
The standards for communications in the United States Navy over a computer-to-computer radio data link are set forth in military standard MIL-STD-188-203-1A dated Jan. 8, 1988 and entitled "INTEROPERABILITY AND PERFORMANCE STANDARDS FOR TACTICAL DIGITAL INFORMATION LINK (TADIL)A" otherwise known as Link-11. A useful discussion of prior art technology and difficulties is also set forth in U.S. Pat. No. 4,199,809.
A Link-11 data message consists of 48 binary digits. The bits are assembled in two frames, each of 24 bits. Six Hamming code parity check bits are added in each frame to make 30 bits for transmission. Each of the 15 pairs of bits in the frame are converted into a phase shift, with shifts of -45.degree., -135.degree., -225.degree. and -315 representing 4 bit pairs, namely (1,1), (0,1), (0,0) and (1,0). Differential Quadrature Phase Shift Keying (DQPSK) is applied to each of 15 audio tones. 14 of the tones are at frequencies of 935 Hz through 2,365 Hz at 110 Hz intervals, with a 15th tone at 2,915 Hz. The transmitted audio signal is the sum of the 15 modulated signals plus a Doppler reference tone of 605 Hz. Therefore, the signal consists of 16 superimposed sine waves, with nominal frequencies given by the tone library of 605, 935, 1045, 1155, 1265, 1375, 1485, 1595, 1705, 1815, 1925, 2035, 2145, 2255, 2365, and 2915 Hz. It is important to note that all of the 16 frequencies are separated from each other by 110 Hz or an integer multiple thereof. The Doppler reference tone of 605 Hz is used for Doppler shift corrections.
The transmitter at the NCS uses the audio signal to amplitude modulate an HF radio frequency carrier and transmit a double sideband suppressed carrier HF signal. Each of the two sidebands, the upper and the lower, contain the same audio signal upon transmission, but anomalies in the propagation path, noise, and interference may cause the received sidebands to differ.
Each Link-11 message is made up of a preamble consisting of five frames which contain only 605 Hz and 2915 Hz tones, followed by an arbitrary number of data frames containing all 16 tones. During the five frame preamble, the 2915 Hz tone is given twice its normal amplitude and is phase shifted 180.degree. at each frame boundary to provide for frame synchronization. The 605 Hz tone is given twice the amplitude of the 2915 Hz tone during the preamble frames and twice the amplitude of the information carrying tones for all other transmitted frames. The 6th frame is a reference frame during which all data tones are transmitted to establish a reference phase for each. Frame positions 7 and 8 provide a start code control signal indicating that data will follow. After the data is transmitted, a picket stop code control signal is transmitted to complete the message.
Link-11 signals are divided into fixed-length frames having a duration of 13.33 msec (fast) or 22 msec (slow), at the operator's option. Within individual frames, each tone has a fixed phase. With the exception of the 605 Hz tone, which is unmodulated and used for Doppler correction, the phase of each tone changes at each frame boundary. At the fast speed of 13.33 msec, the throughput is 2250 bits per second or 47 bits per frame.
Differential Quadrature Phase Shift Keying (DQPSK) is used to encode data, so that the phase change of each tone at the frame boundary encodes a 2-bit value depending upon the quadrant in which the differential phase value occurs.
The protocol of Link-11 includes 5 modes of operation namely, Roll Call, Net Sync, Net Test, Broadcast and Short Broadcast. Further discussion of the protocol may be found in military standard. MIL-STD-188-203-1A referred to previously.
As described in military standard MIL-STD-188-203-1A, the multi-tone Link-11 waveform is usually transmitted over the HF band (2-30 MHz) or the UHF band (200-400 MHz). In the case of UHF, the waveform frequency modulates the carrier (FM) while at HF the modulation is single side band AM (SSB). In the latter case, a frequency shift of the carrier due either to relative motion induced Doppler shift or relative miss-tuning of the transmitter with respect to the receiver, results in a uniform translation of all frequency components of the audio waveform. See, in particular, Sections 5.1.1 and 5.1.2 of military standard MIL-STD-188-203-1A. Regardless of its physical causes, this uniform frequency shift is generally referred to as "Doppler shift." Therefore, with respect to a conventional multiple tone Link-11 waveform, this means that all 16 tones, including the unmodulated 605 Hz Doppler reference tone and the 15 phase modulated data tone (935 to 2915 Hz) will all undergo an identical uniform frequency shift, f.sub. DP. For further discussion, see sections 5.1.10 and 5.2.7 of military standard MIL-STD-188-203-1A. The aforementioned shift f.sub.DP, which can be as great as + or -75 Hz, affects the entire signal and must be removed or large data errors will occur.
The patent literature describes the difficulties with earlier approaches. In particular, U.S. Pat. No. 4,199,809 describes clearly how the prior art hardware equipment is highly customized and difficult to maintain. The size of prior art Link-11 systems is relatively large and generally requires chilled water cooling. Changes to equipment are difficult to implement and costly to obtain.
The use of Hilbert transforms in the context of a Link-11 system is discussed in U.S. Pat. No. 4,076,956. Other U.S. Patents relating to the correction of Doppler shift in the context of Picket systems include U.S. Pat. Nos. 4,719,468; 4,875,050 and 4,876,546.
Phase correction is also a problem in Link-11 systems and is discussed along with Fast Fourier Transforms (FFT), and frame boundary sync detection in U.S. Pat. No. 4,199,809. Phase correction in a quadrature-like phase shift signal is discussed in U.S. Pat. No. 4,707,839.
The invention arose in the context of the above-described prior art.